1. Field of the Invention
This invention relates generally to patterned perpendicular magnetic recording media, such as disks for use in magnetic recording hard disk drives (HDDs), and more particularly to patterned-media disks wherein the data bits are stored on elevated magnetic data islands or pillars isolated from one another by recessed trenches.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks such that there is a single magnetic domain in each island or “bit”. The single magnetic domain can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional continuous media wherein a single “bit” may have multiple weakly coupled magnetic grains separated by grain boundaries. Patterned-media disks may be perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define troughs or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with simpler fabrication processes which avoid etching of the magnetic recording layer material. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. It was believed that because the trenches were recessed they would be far enough from the read/write head to not adversely affect reading or writing. This type of patterned media is described by Moritz et al. in “Patterned Media Made From Pre-Etched Wafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp. 1731-1736. This type of patterned media with perpendicular magnetization on the pillar ends provides opportunity for ultra-high density magnetic recording.
Like conventional non-patterned or continuous-media disks, patterned-media disks also have nondata servo regions that are used for read/write head positioning. The servo regions in the pre-etched type of patterned-media disks with elevated spaced-apart data pillars are also patterned and thus contain elevated nondata servo islands or pillars that are separated by trenches. The servo pillars are “servowritten” or pre-magnetized during the manufacturing process and are not intended to be rewritten during normal operation of the HDD. The proposed method for servowriting this type of disk is to DC “erase” the disk during manufacturing with a large magnet, leaving all of the servo pillars magnetized in the same direction. Thus for a patterned-media perpendicular magnetic recording disk, all of the servo pillars have the same magnetization direction, i.e., either “into” or “out of” the surface of the disk. Because only a single polarity of magnetization is used, half of the available signal amplitude from the servo pillars is sacrificed and thus the signal-to-noise ratio (SNR) of the servo signal is less than optimum.
What is needed is a patterned-media perpendicular magnetic recording disk with patterned nondata servo regions that provide a servo signal with improved SNR.